L7 Heart- Cardiac Refractory Periods

Question 1

Define absolute/effective refractory period

Answer 1

Channels responsible for the AP upstroke are completely inactivated, so no APs can be elicited

Question 2

Define relative refractory period

Answer 2

Channels responsible for the AP upstroke are partially recovered, so abnormal APs can be elicited

Question 3

Explain the cellular mechanism responsible for absolute and relative refractory period.

Answer 3

They are due to voltage- and time-dependence of Na (fast response) and Ca (slow response) channels

Question 4

Compare and contrast refractory characteristics of slow and fast APs

Answer 4

Fast response- short, primarily voltage-dependent refractoriness, as soon as repolarized it’s ready again
Slow response- primarily time-dependent refractoriness, still refractory even after repolarization (only Ca channels= long reactivation)

Question 5

Describe R on T phenomena

Answer 5

Premature beat (R wave) that occurs during the relative refractory period (T wave) of the previous beat

Question 6

What is the pathological signifigance of R on T?

Answer 6

PVCs sustaining during the relative refractory period can put the heart into V Tach

Question 7

Explain how refractory characteristics of AV node “protect” the ventricle from AFib

Answer 7

Refractory period of a slow response is longer than the AV node AP- even though the voltage of the AP fully repolarizes, the cell is still refractory b/c the refractory period of slow Ca channels is more dependent on time than voltage
-Conduction through AV node slows when stimulated at higher rates

Question 8

Explain what determines the ventricular rate and rhythm during AFib

Answer 8

• Rate determined by the AV node refractory period

- Rhythm determined by if an impulse hits the AV node when it is in a refractory period

Question 9

Explain the effect of heart rate on AP duration (interval-duration relationship)

Answer 9

As HR increases, AP duration (systole) decreases

Question 10

How does interval-duration relationship affect the QT interval on the EKG?

Answer 10

AP duration= QT interval, so increased HR= decreased QT interval

Question 11

Define prolonged QT syndrome

Answer 11

Abnormal prolongation of QT interval

Question 12

What are the potential causes of prolonged QT syndrome?

Answer 12

Acquired- bradycardia, hypokalemia, drugs (quinidine)

Congenital- due to genetic lesions in Na and/or K channels

Question 13

What is torsades de pointes?

Answer 13

Polymorphic VTach

-Results from prolonged QT conditions, possibly from development of early afterdepolarizations